N A N O E X P R E S SModification of alumina matrices through chemical etching and electroless deposition of nano-Au array for amperometric sensing Arunas Jagminas Æ Julijana Kuzmarskyte
Trang 1N A N O E X P R E S S
Modification of alumina matrices through chemical etching
and electroless deposition of nano-Au array for amperometric
sensing
Arunas Jagminas Æ Julijana Kuzmarskyte˙ Æ
Gintaras Valincˇius Æ Luciana Malferrari Æ
Albertas Malinauskas
Received: 29 December 2006 / Accepted: 26 January 2007 / Published online: 2 March 2007
to the authors 2007
Abstract Simple nanoporous alumina matrix
modifi-cation procedure, in which the electrically highly
insu-lating alumina barrier layer at the bottom of the pores is
replaced with the conductive layer of the gold beds, was
described This modification makes possible the direct
electron exchange between the underlying aluminum
support and the redox species encapsulated in the
alu-mina pores, thus, providing the generic platform for the
nanoporous alumina sensors (biosensors) with the
direct amperometric signal readout fabrication
Keywords EIS Modification morphology
Nanoparticles Porous alumina
Introduction
Porous anodic oxide films of aluminum anodically
grown in the solutions of oxalic and/or sulfuric or
phosphoric acids have been used for decades as
pro-tection and hard coatings or adhesive layers In recent
years these films, so-called alumina, due to their honeycomb high-ordered and well-predetermined structure, showing tube shaped pore array with a center-to-center spacing from few tents to about
550 nm [1,2] and the pore diameter from about 10 to
250 nm [3], are widely used as a host material for fabrication nanostructured arrays of metals, [4 6] semiconductors, [7 9] conducting polymers, [10] and carbon tubes [11, 12] Notably, that high-ordered alumina matrices filled with nanowires or nanotubes of desired material are promising candidates for catalyst, [13] functional electrodes, [14] future sensors, [15,16] magnetic, [17] and optoelectronic [18, 19] devices Furthermore, high-ordered alumina membranes recently have been used for detection DNA sequences
at the nmol cm–2 level, [20] preparation of new bio-chemical reactor systems, [21] and the synthesis of nano-black lipid membranes [22] The use of anodized aluminum electrodes as support for amperometric sensors is, however, unexplored due to high resistance
of alumina a thin scalloped barrier-oxide layer sepa-rated the thick porous one from the metal [23] that is a key problem On the other hand, the development of such system within the porous alumina matrix may lead
to construction of novel redox biosensor configura-tions In present paper, we describe a simple nano-porous alumina matrix modification procedure, in which the electrically highly insulating barrier layer at the bottom of the pores is replaced with the gold beds This modification makes possible the direct electron exchange between the underlying aluminum support and the redox species encapsulated in the alumina pores, thus, providing the generic platform for the nanoporous alumina sensors (biosensors) with the direct amperometric signal readout
Electronic supplementary material Supplementary material is
available in the online version of this article at (doi: 10.1007/
s11671-007-9043-2 ) and is accessible for authorized users.
A Jagminas (&) J Kuzmarskyte˙ A Malinauskas
Institute of Chemistry, Gosˇtauto 9, 01108 Vilnius, Lithuania
e-mail: jagmin@ktl.mii.lt
G Valincˇius
Institute of Biochemistry, Mokslininku˛ 12, 08412 Vilnius,
Lithuania
L Malferrari
Instituto Nacionale di Fisica Nucleare, viale Berti-Pichat 6/2,
40127 Bologna, Italia
DOI 10.1007/s11671-007-9043-y
Trang 2Experimental details
Several different aluminum sheets, which purity ranged
from 98.0 to 99.99% (Goodfellow, Cambridge Ltd.),
were tested as precursors for the porous anodic oxide
film fabrication The samples in the form of the
flag-shape plates (7 · 7 · 0.2) mm were annealed at 500 C
for 3 h, chemically cleaned, rinsed, and electropolished
before use, as usually Porous oxide films of from 3 to
10 lm thick were grown under the anodizing cell
voltage control in either an aqueous oxalic (0.3 M;
17 C; 40 V) or phosphoric (0.04 M; 16 C; 150 V) acid
solution To destroy the insulating barrier oxide layer
only at the bottom of pores, several electrochemical
and chemical etching steps were used The alumina
nanoporous layer modification included: (i) stepwise
decrease of anodizing voltage (Ua) at the end of the
film growth down to Ua,fin; (ii) chemical etching in a
solution of 0.5 M phosphoric acid at 30 C for time sw
and (iii) electroless deposition of zinc/nickel layer
in the immersion solution of zinc and nickel
fluorbo-rates (0.17 M Zn(BF4)2+ 0.87 M Ni(BF4)2+ 0.38 M
NH4BF4) at room temperature for time sim The
completeness of deletion the alumina barriers at the
bottom of pores was checked after each treatment step
using scanning electron microscopy (SEM) (a Philips
30 L microscope equipped with energy dispersed X-ray
spectrometer) and electrochemical impedance
spec-troscopy (EIS) The EIS measurements were carried
out using a Solartron system that includes model 1286
potentiostat and model 1250 frequency response
ana-lyzer (Farnborough, UK) The EIS experiments were
conducted in a frequency range of 1 Hz–100 kHz, with
equal spaced data points on a logarithmic scale and
with ten measurements per decade To avoid nonlinear
responses the amplitude of applied sinusoidal ac signal
was set to 10 mV The spectral data were analyzed/
fitted with ZView software (Scribner Associates, South
Pines, NC, USA)
Electrochemical measurements were carried out
using a three-electrode polystyrene cell (2 ml) with a
6-mm-i.d KalrezTM O-ring, which set up the exposed
to solution surface area of the working electrode
to 0.32 ± 0.02 cm2 A platinum coil (~4 cm2) and
Ag/AgCl/KClsat (Microelectrodes, Inc., Bedford, NH)
were used as the auxiliary and reference electrodes,
respectively EIS measurements were carried out at 0 V
bias versus the reference electrode at 20 ± 1C in
aerated 10 mM sodium phosphate buffer (pH 7.0)
solution containing 100 mM sodium sulphate
For backside observations of the film morphology
the alumina matrices were detached from substrate
by dissolution of aluminum as described by Li et al [1, 11]
Voltammetric behavior of alumina matrices was studded using a PI 50-1 potentiostat (Belarus) inter-faced through a home-made analogue to a digital converter with a PC and a PR-8 programmer (Belarus) All experiments were carried out at a temperature of
20 ± 0.2 C in a conventional three-electrode cell The working electrode was either a vertical Au disc of
1 cm2geometric area, made from a mat polycrystalline
Au sheet (99.99% purity), or alumina/nano-Au/Al of a same geometric area A Pt sheet 3 cm2in area was a counter-electrode and a saturated potassium silver-sil-ver chloride electrode (SCE) was used as a reference
In order to avoid the contamination of the working solution {5 mM K3[Fe{CN)6] + 5 mM K4[Fe(CN)6]} with Cl– ions, the SCE was connected to the electro-chemical cell through a 1 M KCl with agar-agar jelly bridge Prior to each experiment, the working solution was deaerated with argon
All solutions were prepared using highest purity acids, chemically pure salts and Milli-Q water
Reproducibility of the measurements was checked
by 3 repeated experiments
Results and discussion
We found that the quality of perforation of alumina matrices at the bottom of the pores and the ability to form there well-adhered Zn/Ni layer depend strongly
on the aluminum purity as well as on the parameters of post-treatment processes, e.g Ua,fin, sw, and sim No uniform deposition of Zn/Ni layer at the bottom of pores was observed in the case of high purity aluminum electrodes (>99.9%) Instead, good quality immersion
Zn layers were obtained using 99.685% purity alumi-num (Si 0.156; Fe 0.089; Zn 0.03; Mg 0.021; Cu 0.016;
Mn, Ti, Cr and Pb 0.003 wt.%) This is consistent with the experimental facts indicating preferable formation
of Zn/Ni immersion layer on the surfaces plate of aluminum alloys [24]
Figure 1show the cross-sectional and backside SEM images of the oxalic acid alumina matrices grown onto 99.685% purity aluminum at Ua= 40 V for 1.5 h after the perforation of alumina barriers by decreasing
Uadown to Ua,fin= 5.0 V and subsequent etchings in the phosphoric acid and immersion solutions for 22 and
7 min, respectively Notably, all these procedures lead
to the formation of alumina matrix with diameter pores
of ~ 45 nm and the interpore distance of ~ 108 nm without detachment the porous matrix from the
Trang 3substrate The optimal perforation conditions of the
phosphoric acid films included the gradual decrease of
the anodizing end-voltage from 150 V to Ua,fin25–27 V
and the subsequent chemical etching steps for sw= 55–
65 min and sim = 5–7 min resulting in the fabrication
of alumina with average diameter of pores close to
200 nm and the center-to-center spacing of ~ 410 nm
(Fig.2) Notably, all these post-anodizing procedures
lead not only to the perforation of the alumina
nano-channels but also to the deposition of a thin Zn0/Ni0
immersion layer at the aluminum/solution interface at
the bottom of pores Furthermore, seeking to cover the
bottoms of opened pores with well-adherent layer of
precious metal, gold electroless deposition process was
chosen in a 10 mM HAuCl4+ 50 mM MgSO4solution
by experimental way The stored of alumina/Zn/Al
electrodes in this solution leads to the formation of
gold beds by the chemical exchange reaction between
the Au3+ ions and the metallic Zn layer deposited at the places of opened pores:
3Zn0 + 2Auþ3 ! 3Zn2þ + 2Au0 ð1Þ
As a result of this treatment, gray color of the alu-mina matrix acquired during zinc deposition turns into olive signaling of the formation of the nano-Au species This was verified there by recording UV-Vis spectra of the alumina matrices detached from the substrate The spectra in Fig 3B show the emerging of absorbance maximum at 535–550 nm wavelength range, charac-teristic for gold colloids The red-shift of the surface plasmon resonance peak seen as the immersion time increases confirms the growth of nano-Au particles [25]
at the bottom part of the pores In addition, the deposition of gold particles at the bottom of the alu-mina pores has been also visualized by SEM images of the matrix cross-sections (Fig 3A) and EDX analysis data (Fig.3C)
To characterize the electrical properties of alumina matrices, the EIS spectra were taken at each step of alumina formation and modification Figure4 depicts typical Bode plots of the electrode admittance of the aluminum electrode at different stages of the oxide matrix formation and re-construction A systematic variation of the EIS spectra in Fig.4 includes the increase of electrode admittance in the low frequency range and the shift of admittance curves towards lower frequencies The shift of the admittance plots indicates the capacitance increase upon the successive steps of alumina matrix post-treatment, which, as we believe, considerably decreases or even fully removes the insulating barrier The capacitance increase is clearly seen in the complex capacitance curves as well as in the fitting to model [26, 27] parameters, which are pre-sented in the Supporting Information section The most important result that follows from Fig.4 is a noticeable increase of the admittance in the low fre-quency edge of the spectra Particularly, the formation
of the immersion zinc layer at the bottom of the pores yields approximately 3 fold increase, while the gold beds formed in following stage increases the admit-tance by approximately 25–30 times compared to the initial admittance values of the chemically unmodified alumina Moreover, the EIS spectra of pure alumina (Fig 4, curve 1) were not significantly altered by the addition of the redox species to the electrolyte, while the gold-modified alumina matrices exhibited clear sensitivity to the potassium ferrocyanide (curve 4) The redox species especially influenced the low frequency part of the EIS spectra, in which the weight of the Faradaic processes contribution to the EIS signal
Fig 1 The back-side (A) and the cross-sectional (B) SEM
images of alumina matrices grown in a solution of 0.3 M
(COOH)2 at 40 V and 17 C for 1.5 h onto the surface of
99.685 % purity Al followed by decrease of anodizing voltage
down to U a,fin = 5.0 V and subsequent etching in 0.5 M H 3 PO 4
at 30 C for s w = 22 min
Fig 2 The cross-sectional SEM image of alumina matrix grown
in a solution of 0.04 M H 3 PO 4 at 150 V and 16 C for 2 h;
Ua,fin= 27.0 V; sw= 60 min
Trang 4becomes significant In our case, the electrode
admit-tance at 1 Hz increased from ~50 to 150 lS (electrode
surface–0.32 cm2) upon injection of potassium
ferro-cyanide at concentration of 10 mM (compare curves 3
and 4) All this suggests that the alumina modification
procedures used in this work yield nanoporous
elec-trodes, on which the direct electron exchange between
the dissolved redox species and the underlying metal
becomes possible
To probe the direct electron transfer rate we
com-pared the cyclic voltammetry response of the metal
gold and gold-modified alumina electrodes using ferri/
ferro cyanide redox system under the same
experi-mental conditions Figures5A and B illustrate typical
cyclic voltammograms (CVs) obtained in a solution of
10 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) using poly-crystalline Au plate (99,9% purity) and gold-modified alumina electrodes As seen, the shapes of CVs are comparable both qualitatively and quantitatively In particular, potential difference between cathodic and anodic peaks DEp, equals ~200 mV at potential scan rate of 50 mV/s Similar current-potential behavior of the bulk and nanostructured gold electrodes imply that the rate of electron transfer reactions taking place at both electrodes are similar This is an important result because it suggests that the modification route of the nanoporous alumina surfaces presented in this work makes available full removal of the alumina barrier layer from the bottom of the pores
Conclusions Complete deletion of the alumina barrier layer only
at the bottom of the pores can be attained through step-wise decrease of anodizing voltage, several steps of chemical etching and electroless deposition of nano-Au species at the bottom of alumina pores at the aluminum/solution interface By this way, the low resistant nano-Au/alumina/Al electrode for amperometric sensing was fabricated
Fig 3 (A) Cross-sectional SEM image of alumina matrix grown
as in Fig 1 after the additional treatment in the Zn/Ni immersion
solution (pH 6.0) at RT for 5 min and electroless gold plating at
RT for 5 min (B) UV-vis spectra of alumina matrices fabricated
as in (A) on the gold plating time: (1) 0; (2) 2; (3) 5; (4) 15 min.
(C) EDX spectra of alumina matrix grown and re-constructed as
in part B (curve 3)
-6 -5 -4 -3 -2 -1
0.1 10 1000 100000
Frequency, Hz
1 2
3 4
Fig 4 Bode plots of admittance of Al/alumina electrodes within 1–100,000 Hz frequency range corresponding to different stages
of the barrier layer deletion: (1) after anodization of Al specimen
in 0.04 M H 3 PO 4 (U a 150 V; 3.0 h; 16 C followed by step-like voltage decrease to U a,fin 27.0 V and chemical pore widening for
s w 60 min); (2) after formation of Zn0layer at the bottom of the pores by immersion in a solution of Zn/Ni fluorborates for 7 min; (3) after replacement of zinc by gold via chemical exchange reaction; (4) the same as (3), however, the pore-filling solution contains additionally the redox species [10 mM K 4 Fe(CN) 6 ] (vide infra) Electrode surface area exposed to the solution is 0.32 cm 2 Temperature 20 C
Trang 5Notably, the fractal structure of re-constructed
alu-mina matrices results in the specific EIS response that
can be modeled by two parallel CPEs one of which
exhibits a 1 and an another a 0.5
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-4
-2
0
2
4
0,05 V/s 0,02 V/s 0,01 V/s
A
B
E, V
E, V
-4
-2
0
2
4
0,05 V/s 0,1 V/s 0,02 V/s 0,01 V/s
Fig 5 Cyclic voltammograms of the gold plate (A) and
nano-Au/alumina/Al (B) electrodes fabricated as in Fig 1 in a
deaerated and unstirred 10 mM K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] (1:1)
buffered solution (acetate buffer; pH = 6.0) on the potential scan
rate The apparent surface area of electrodes 0.5 cm2